Light Emitting Diodes (“LEDs”) have several advantages for outdoor lighting over High Intensity Discharge (“HID”) light sources, such as long life, lower energy consumption, durability, cold weather performance, directional orientation of beam patterns, instant on/off and controlled dimming without color change. In particular, the directional nature of an LED light source provides the ability to create asymmetric light beam emission patterns by orienting and directing multiple LED light engines that are mounted within the fixture in a specific direction. This eliminates the need to use reflectors and focusing lenses as in conventional light sources.
A major concern, however, when designing an LED outdoor fixture, is effective heat management. The heat that is generated at the semiconductor domain junctions is the primary determinant of the life of the LED and in maintaining a consistent wavelength. LEDs function better and last longer at cold or cool temperatures, and deteriorate more rapidly with increased heat. The design effort to draw heat away from the junctions has often resulted in the LED circuit boards being attached to a finned heat sink, with natural air convection or fans used for cooling. In an outdoor light fixture, however, the ambient temperature may at times be relatively high, even at night. There may also be little natural air movement in or around the fixture, especially in closed-dome fixtures, thus providing little ventilation within the fixture. As such, there is limited convective heat transfer.
In addition to the utilitarian design considerations, outdoor pole luminaries are often decorative pieces, mimicking ornate gas lanterns and early incandescent street and park lighting. Some of the fixture housings are very ornate and incorporate architectural design elements of the locale or historical periods. Replacing the light source in such fixtures with LED lighting is a particularly challenging task.
A major challenge in devising LED lighting fixtures is to ensure that the light from the LEDs is directed effectively and efficiently where it is desired. In particular, for street lighting, a light distribution pattern that extends along the street, and to a lesser extent out into the street, is often desired, with little or no light going back onto street-front properties behind the lamp or upwards.
One solution to such lighting challenges is described in U.S. Pat. No. 8,104,929 (“the '929 patent”) which is directed to an outdoor lighting fixture that uses LEDs as a light source. The fixture includes a decorative housing with a cast metal dome roof having an inner surface and an outer surface. A plurality of LED light engines are provided, with each light engine containing a plurality of LEDs mounted on a circuit board, mounted on a conductive metal substrate. The substrate has a surface opposite the circuit board that is in conformal contact with a portion of the inner surface of the roof of the light fixture. The roof provides increased heat sink mass and the outer surface of the roof provides a heat dissipative surface area outside of the housing. Two of the LED light engines are placed along the inside walls of the dome roof, with one engine located at an acute angle to a line through the housing and the other engine at its complementary angle to the line, such that the light from the two LEDs is emitted in lobes that cross within the fixture and expand outside of it to create an asymmetric overall light profile.
While the design in the '929 patent provides a vast improvement over the prior art fixtures, there is still scope for a further improved fixture configuration that could be used with a center lamppost mounting system where lighting needs to be directed outward and downward, but not upward. A particular challenge is to provide a luminaire that can be used with an “acorn globe,” which has a transparent top, and still comply with recent requirements to limit upward light emission.